RGBa images can have a different transparency level for each individual pixel in the image. This is achieved by adding an alpha channel to the three basic R, G and B channels. The effect is also known as alpha transparency. Compressed images can also utilized alpha transparency by choosing an image channel to be used as the alpha channel via the View - Display Options dialog.
The appearance of each pixel in RGBa images is defined by four numbers: the three R, G and B numbers that specify the color and an alpha channel number that specifies the transparency of that particular pixel in a range from 0 (fully opaque) to 255 (completely transparent). The ability to work with RGBa images is one of the key capabilities that distinguishes a professional-class image editor like Adobe PhotoShop or Manifold System from consumer-grade graphics editors.
If you have not yet done so, please read the Images and Channels topic before proceeding with this topic.
To use RGBa Pixel Transparency:
1. Convert the image to RGBa using Image - Convert to
2a. Paint any gray color into the alpha channel of the image using any paint tool; or
2b. Load a grayscale mask for the alpha channel using Edit - Load Mask/Channel
RGBa pixel transparency works only with RGBa images. If the image is converted to any other type (RGB, grayscale or palette) the partial transparency effects will be lost.
Alpha transparency does not change the R, G, and B channels that define the color value of the pixel. When we appear to "erase" pixels using alpha transparency the pixels and their original color values are still there. They are simply transparent because their alpha channel values have been set to high amounts. If we change our minds about any transparency effects applied we can restore the original appearance of the pixels by converting the image to an RGB image.
See the Painting within Channels topic for examples of painting gray color into the alpha channel to control alpha transparency.
Example
Suppose we have an image of Europe using mostly blue colors.
We can make a copy of the blue Europe image and use Hue / Saturation to change the color range to red.
Using the partial eraser we can erase most of the image leaving only the central portion with a gradual transition from fully opaque pixels to fully transparent pixels.
If this image is layered in a map above the blue Europe image we get the effect seen above.
We can also use masks, such as the grayscale gradient above, to control alpha transparency.
When the mask above is loaded into the alpha channel of the red Europe image it causes zero transparency in black regions of the mask and complete transparency in white areas of the mask. The regions of the mask that transition from black to white cause a gradient of partial transparency in the red Europe image.
Seen in the map above the blue image the result is a gradual color transition from Northwest to Southeast.
There is no limit to the artistic and presentation effects made possible by alpha transparency. We can adjust the region of transparency using a wide variety of Manifold editing tools.
The illustrations above show the red Europe image as a separate image to make clear what is going on. Many times we will edit alpha transparency free-hand in maps using editing tools such as the partial eraser to see the effect in a many-layered map in real time.
We can create many special effects by making copies of an image and then adjusting the different copies in different ways (such as by Hue / Saturation or other commands), partially erasing pixels in the variations and then recombining them.
Uses for Alpha Transparency
· Creation of complex images using many layers.
· Graphics art effects. See the Invisible Pixels topic for an example.
· Drop shadows, halos and "stand outs". See the Gaussian Blur topic for an example.
· "Feathered" transitions from one image to another.
· Color range effects as shown above.
· Transitions between different effects, such as blur to non-blurred images.
· Fine feathering of images to eliminate anti-aliased pixels at the edges of "cut outs."
· To give the appearance that text or other images are painted onto a surface.
· To merge tiles of different images into one image without obvious borders.
· To paste patches into images to hide defects or unwanted imagery without an obvious border line.
Example
Manifold System has such powerful image manipulation capabilities that it may also be used to create complex images by combining multiple images in layers as is done with professional quality graphics image editors such as Adobe PhotoShop.
The marketing promotion "splash" screen used with Manifold System Release 5.00 was created using over 40 layers of other images. Alpha pixel transparency and invisible pixels were used throughout the various layers so lower layers could combine to form the image. Looking at seventeen of the various layers we can see how the image was created:
The bottom three layers were screen shots from a Terrain window using different settings for sky and surfaces. The uppermost of these is the grid.
The next two layers are the floating city and its shadow, which are in separate layers so that the transparency and relative position of the shadow may be easily controlled. Once the shadow was in the right position relative to the city, the Move Together box for both was clicked for these two layers so that moving the city automatically moved the shadow as well. See the Gaussian Blur topic for details on creating drop shadows.
The next twelve layers consist of text or logotype images together with various drop shadows. By checking and unchecking the Move Together boxes as we click on any of the layers, we can move various layers together to compose the image as desired.
The actual production "map" for the splash screen included over 40 layers. The extra layers contained variations of the logotype, different visual elements such as different sky cloud patterns and other variations. By checking the layers ON and OFF and moving the various layers about it was easy to try out many different variations to end up with exactly the effect desired.
Each image in the layer stack was heavily manipulated using various effects, resized, partially erased or made transparent to "feather" its features visually into features from other layers, and so on. For example, the city originated in a scanned photograph snapped from the bell tower in St. Mark's square in Venice. It was cut out of the photograph and then different parts of the image had hue, contrast and color balance adjusted in different ways. To bring out detail in the trees the green areas were re-built from several layers using different settings for brightness, contrast, saturation, colorization and color balance.
See the Composing Complex Images in Layers topic for an additional example of how to compose complex images using Manifold.
Saving the Alpha Channel as a Separate Image
At any time we can save the separate channels of an RGB or RGBa image as different grayscale images. To do so, we use the Edit - Save Mask/Channel command with the channel to be saved specified in the dialog's Save box. For example, we can save the gradations of an alpha channel as a grayscale image by choosing Channel: alpha in the dialog's Save box. This is a very useful way of saving different transparency patterns. We can try out different amounts of alpha transparency as we like and always return to one pattern that we like the best.
See the Masks topic and the Separating Images by Channels topic for information on how Edit - Save Mask/Channel can be used to separate an image into different images for each channel.
Invisible Pixels vs. RGBa Pixel Transparency
Alpha transparency is a different mechanism than invisible pixels . Key differences include:
· Invisible pixels are an "all or nothing" effect: either the pixel is fully visible or it is invisible. Alpha transparency allows a different transparency setting for each individual pixel.
· Deleting a pixel to make it an invisible pixel is a permanent deletion. The pixel with its original color values cannot be retrieved. Alpha transparency, in contrast, can be changed at any time to restore the original appearance of pixels.
· Using invisible pixels does not add to the size of the image. Alpha transparency works only with RGBa images and requires a fourth channel for each pixel whether transparency is used or not.
· Invisible pixels do not take any longer to process than regular pixels. RGBa images take longer to display because transparency for each individual pixel must be reckoned.
Performance Considerations
Working with RGBa images is slightly slower than working with regular RGB images because of two factors:
· RGBa images are larger than RGB images.
· Adjusting transparency for each individual RGBa pixel takes longer than simply displaying an RGB pixel.
Most graphics art editing of images is done with images that are tens or hundreds of times smaller than images can be in GIS applications. For that reason using RGBa and alpha transparency is not normally a performance issue with graphics arts image editing in applications such as Adobe PhotoShop.
Because GIS images such as raster data images can be well over a hundred megabytes we need to consider in a GIS environment whether it is worth it or not to convert images to RGBa and to use alpha transparency. Operations with such large images on desktop machines should consider every performance issue.
Nomenclature
Transparency and opacity are two terms that mean the same concept viewed from different directions. When something is completely opaque it is not at all transparent. When something is perfectly transparent it may be said to have zero percent opacity.
Which word is used depends on the discussion. When imagining layers stacked up above each other like transparent sheets it is conceptually clearer to use the word transparency. When discussing a specific percentage of light transmission to be applied via a slider bar in a dialog most applications use the word opacity.
The convention in the graphics arts editing software industry is to adjust layer transparency with controls that set a number from 0% to 100% opacity, so that an image with 100% will be fully opaque and not allow any view of an image underneath it. Manifold follows this convention. This convention persists in the graphics arts industry even though the technical implementation of transparency effects is done using an alpha channel within RGBa images where the higher the value of the alpha channel (from 0 to 255) the higher the transparency.
One therefore encounters the slight conceptual dissonance of increasing opacity with higher numbers (up to 100%) in dialogs and other user interfaces while the internal data sets use numbers (alpha channel values) in which opacity decreases with higher numbers. Since we rarely set alpha values by hand this is not so bad. Alpha values are normally set using various tools, such as erasers, or masks. In the case of masks, the darker the mask region the lower the alpha value is and thus the higher the opacity. From a casual conceptual view this is very acceptable because it leads to an effect where black regions of masks cause full opacity and white regions of masks cause full transparency. Since we are used to thinking of "white space" as being transparent this works well as a natural mnemonic for the effects of masks.
See Also
Use the Layers pane with images to turn on a background color layer or to show a border around an image.